Sheet-shaped materials comprised primarily of a fibrous substrate and polyurethane have excellent characteristics that cannot be found in natural leather, and are widely used in various applications. In particular, leather-like sheet-shaped materials using a polyester fibrous substrate have excellent light resistance, and thus have been increasingly applied to clothing, chair upholstery, automobile interior materials and other applications.
Such sheet-shaped materials are typically produced by wet coagulation process involving impregnating a fibrous substrate with a polyurethane solution in an organic solvent and then immersing the resulting fibrous substrate in a solvent that does not dissolve the polyurethane (i.e., water or a mixed solution of an organic solvent and water) to coagulate the polyurethane. The organic solvent used to dissolve the polyurethane is a water-miscible organic solvent such as N,N-dimethylformamide (DMF). There has been proposed, for example, a process comprising adding polyvinyl alcohol (hereinafter, also called “PVA”) to a nonwoven fabric, impregnating the nonwoven fabric with a polyurethane solution in DMF, wet-coagulating the polyurethane in a 45% aqueous DMF solution, and removing the PVA in hot water (see Patent Literature 1). However, since organic solvents are typically highly harmful to human bodies and the environment, there has been a strong demand for a production process of a sheet-shaped material not using organic solvents.
In order to provide a specific solution to meet the demand, there has been proposed, for example, a process using a waterborne polyurethane (polyurethane dispersed in water) in place of a conventional polyurethane in an organic solvent. However, a sheet-shaped material produced by impregnating a waterborne polyurethane into a fibrous substrate has a hard texture. This problem is mainly caused by strong adhesion of the polyurethane to the fibers in the fibrous substrate. To solve the problem, there has been proposed a process which comprises the following steps as in a conventional production process using a polyurethane in an organic solvent: first adding PVA to a fibrous substrate for the purpose of partially suppressing the adhesion between the fibers and a polyurethane to be added later and thereby of forming voids between the fibers and the polyurethane, adding the polyurethane, and removing the PVA (see Patent Literature 2). However, since PVA is water soluble, when the fibrous substrate with added PVA is wetted with water, the PVA is dissolved away and lost in the water. Regarding this, the process in Patent Literature 2 involves the following steps in which the fibrous substrate is wetted with water: (i) the step of impregnating the fibrous substrate with a waterborne polyurethane and (ii) the step of ultra-fining the fibers with an aqueous alkali solution. In the latter step of ultra-fining the fibers, the dissolution of PVA is prevented by the addition of borax to the aqueous alkali solution. However, in the former impregnation step with the waterborne polyurethane, the dissolution of PVA in the waterborne polyurethane dispersion cannot be prevented because the PVA used (degree of saponification: 98%) has a low degree of polymerization of 500. If the PVA is dissolved away in the waterborne polyurethane dispersion, the adhesion of the polyurethane to the fibers cannot be stably controlled, resulting in a sheet-shaped material having a hard texture.